Polyaryl ether ketones are well-known high-performance polymers. They are used for applications with temperature or mechanical constraints, or even chemical constraints. These polymers are found in fields as varied as aeronautics, offshore drilling and medical implants. They may be used for all the technologies in which thermoplastics are used, such as molding, compression, extrusion, spinning, dusting or laser prototyping. In the latter case, it is necessary to have a powder of controlled diameter and morphologies, and also low contents of residual products, whether they are monomers or solvents, in order to minimize their deposition onto the various optical members of the prototyping machine. In addition, good heat stability is sought in order to allow the recyclability of the powders in this prototyping process. Two synthetic routes are used for preparing polyaryl ether ketones. On the one hand is a “nucleophilic substitution” process, for which access to the monomers is difficult since it is necessary to prepare special fluoro or chloro monomers. The synthetic conditions of the nucleophilic substitution process are also difficult (350-400° C. in diphenyl sulfone), and the post-reaction treatments are constraining (difficult removal of the salts and of the solvent).
On the other hand is the “electrophilic substitution” process, which may be performed at either high temperature or room temperature. The advantage of this second process lies in the possibility of polymerizing at moderate temperature (−20° C. to 120° C.), which limits the side reactions. Moreover, both the monomers and the solvents are more industrially available.
The latter process is widely described in the literature, for instance in U.S. Pat. Nos. 4,841,013, 4,816,556, 4,912,181, 4,698,393, WO 95/00446, WO 4 716 211, WO 2011/004 164 or WO 2011/004 164.
The reaction is an electrophilic substitution between one or more aromatic acid chlorides and one or more aromatic ethers in the presence of a Lewis acid. It takes place in a solvent, occasionally in the presence of a dispersant (U.S. Pat. No. 4,698,393, WO 95/00446) and generally takes place in two stages with a first phase at room temperature or even below 0° C., and the reaction is then completed at a temperature of between 0° C. and 120° C. depending on the solvent. The process may also be performed at higher temperature, but this route generates more side reactions. The reaction mixture is then treated with a protic compound to extract all or part of the Lewis acid. The choice of the protic compound depends on the solvent used. In WO 4 841 013 and WO 2011/004 164, U.S. Pat. Nos. 4,716,211, 4,912,181 or WO 2011/004 164, the solvent used is dichloromethane and the protic compound is water. In U.S. Pat. No. 4,716,556 and WO 95/00446, the solvent is ortho-dichlorobenzene and the protic compound is methanol.
It is in point of fact a matter of having sufficient solubility of the protic compound in the solvent so that it can either react with AlCl3 and/or decomplex it from the polymer; for example, water has a solubility in dichloromethane of 0.2% by weight at 20° C.
The Applicant has now discovered that not only is it possible to use a protic compound that is very sparingly soluble in the solvent, but also that this choice makes it possible to obtain a polymer that is more stable when the protic compound is water. In the present invention, the solvent used is aprotic, preferably ortho-dichlorobenzene, but use may also be made of difluorobenzene, trichlorobenzene or a mixture thereof, and the protic compound is water or acidic water, which dissolve in only very low amounts, typically 0.015%, in ortho-dichlorobenzene. The advantage of avoiding the use of an alcohol makes it possible to avoid reactions of the alcohol on the chains of the polyaryl ether ketones and thus to have better stability. In the case of dichloromethane, water is used as protic compound, its action is incomplete since the polymer is obtained in the form of a bulky gel and does not make it possible to be treated correctly with the water within industrially reasonable times without a mechanical treatments that is difficult to perform, or certain additives must be used in order to allow dispersion of the PAEK in the solvent and thus to ensure a sufficiently effective action of water at the end of polymerization. In addition, the proportion of solid (mass of the polymer/mass of solvent) typically used in the invention may be brought to values ranging up to 10%, which is not possible with dichloromethane without being confronted by a bulky gel without use of dispersant. Another advantage consists in using water to perform an azeotropic entrainment of the solvent. A step of finishing by drying under vacuum preferably at 30 mbar at a temperature above the glass transition temperature (Tg) of the polymer, preferably at least 10° C. higher (Tg+10° C.) and more particularly Tg+30° C., Tg measured by DSC, ensures removal of the residual materials, in particular the solvent. The polyaryl ether ketones prepared according to the present invention have very low contents of residual materials and of residual acidity, and may thus be advantageously used in laser sintering processes, minimizing the fouling of optical systems, and with good recyclability.
Although, in document U.S. Pat. No. 4,698,393, use is made of a combination of ortho-dichlorobenzene as solvent and acidic water as protic compound, this treatment step is only partial since it is followed by a second treatment with methanol, which leads subsequently to side reactions. Furthermore, a dispersant is used during the synthesis, which is not the case in the present invention.